1. Field of the Invention
The present invention relates to a distributed optical fiber sensor which uses an optical fiber as a sensor, and which is capable of measuring the strain and temperature in the longitudinal direction thereof with high precision.
2. Description of the Related Art
Conventionally, as technology of measuring the strain and temperature, there is a method based on the Brillouin scattering phenomenon which occurs in the optical fiber. In this method, the optical fiber is used as the medium from which the strain and/or temperature is to be detected in the environment (object to be measured) where such optical fiber is to be installed.
The Brillouin scattering phenomenon is a phenomenon where power travels via the acoustic phonon in the optical fiber when light enters the optical fiber, and there are the stimulated Brillouin scattering phenomenon which occurs as a result of two lights with mutually different frequencies entering the optical fiber and based on the interaction of the two lights, and the natural Brillouin scattering phenomenon which occurs as a result of light entering the optical fiber and based on the interaction of the foregoing light and the acoustic phonon that is generated by the thermal noise in the optical fiber. The Brillouin frequency shift that is observed during the Brillouin scattering phenomenon is proportional to the sonic velocity in the optical fiber, and the sonic velocity is dependent on the strain and temperature of the optical fiber. Thus, the strain and/or temperature can be measured by measuring the Brillouin frequency shift.
As representative systems for measuring the distribution of strain and temperature using the Brillouin scattering phenomenon, there are BOTDA (Brillouin Optical Time Domain Analysis) and BOTDR (Brillouin Optical Time Domain Reflectometer).
Foremost, with the BOTDA, the stimulated Brillouin scattering phenomenon is used, two laser beams with mutually different frequencies enter the detection optical fiber in an opposing manner as pump light and probe light, and the optical intensity of the light pertaining to the stimulated Brillouin scattering phenomenon output from the end, of the detection optical fiber, into which the pump light entered is measured in the time domain. With the BOTDA, the acoustic phonon is excited based on the interaction of the pump light and the probe light.
Meanwhile, with the BOTDR, once laser beam enters as pump light from one end of the detection optical fiber, light pertaining to the natural Brillouin scattering phenomenon output from the one end is detected by an optical band pass filter, and the optical intensity of the detected light pertaining to the natural Brillouin scattering phenomenon is measured in the time domain. With the BOTDR, the acoustic phonon generated by the thermal noise is used.
Subsequently, with the BOTDA and the BOTDR described above, this kind of measurement is performed for each frequency while sequentially changing the frequency of the pump light or, in the case of the BOTDA, the frequency of the probe light, the Brillouin gain spectrums (or Brillouin loss spectrums in the BOTDA) of the respective portions along the longitudinal direction of the detection optical fiber are respectively obtained, and the strain distribution and/or temperature distribution along the longitudinal direction of the detection optical fiber is measured based on the foregoing measurement result. As the foregoing pump light, under normal circumstances, a light pulse with a rectangular optical intensity is used, and, as the probe light in the BOTDA, continuous light (CW light) is used.
Here, with the BOTDA, while the Brillouin gain spectrum is detected by causing the frequency of the pump light to be higher than the frequency of the probe light with the probe light as the reference on the one hand, the Brillouin loss spectrum is detected by causing the frequency of the probe light to be higher than the frequency of the pump light. Moreover, with the BOTDR, the Brillouin gain spectrum is detected. With the BOTDA, the strain and/or temperature can be obtained by using either the Brillouin gain spectrum or the Brillouin loss spectrum. In this specification, the Brillouin gain spectrum and the Brillouin loss spectrum are simply referred to as the “Brillouin spectrum” as appropriate when referring to the BOTDA.
The spatial resolution of the BOTDA and the BOTDR is restricted by the pulse width of the light pulse of the pump light that is used for the measurement. Although the speed of light in the optical fiber will differ slightly depending on the material of the optical fiber, with a standard optical fiber that is generally used, it takes approximately 28 ns for the complete rise of the acoustic phonon. Thus, the Brillouin spectrum is a Lorentzain curve up to where the pulse width of the light pulse is approximately 28 ns or more, and, if the light pulse width is made shorter than the above, it becomes a wide band curve, and takes on a smooth shape that lost its steepness in the vicinity of the center frequency. Thus, it becomes difficult to seek the center frequency, and the spatial resolution thereof is usually said to be approximately 2 to 3 m.
Thus, the present inventors proposed, in Brochure of International Publication No. 2006/001071, a method of measuring the distribution of the strain and/or temperature with high precision (for example, 200με or less) and high spatial resolution (for example, 1 m or less) by configuring the foregoing light pulse from two components. The present inventors refer to this system as the PPP-BOTDA/BOTDR (Pulse Pre-Pumped BOTDA/BOTDR). Note that 100με corresponds to 0.01% (100με=0.01%). Moreover, the Brillouin frequency shift is approximately 500 MHz/% relative to the strain.
Nevertheless, since the Brillouin frequency shift amount that is measured using the Brillouin scattering phenomenon changes depending on the two parameters of strain and temperature of the optical fiber, the parameter that can be measured using the Brillouin scattering phenomenon is basically one of either the strain or temperature, and it is not possible to separately and simultaneously measure the strain and temperature.